The present invention relates to a device, such as pipes and various valves used with such pipes, which is made of carbon steel and constitutes the wet steam system, the feedwater and condensate system, and the drain system in thermal and nuclear power plants and which is protected from reduction of its wall thickness due to erosion-corrosion, a special and rapid form of damage to metal parts.
Carbon steel, such as forged steel and cast steel, is widely used as a material suitable for compositional parts, such as piping and various valves including gate valves, globe valves and check valves, used in the wet steam system, feedwater and the condensate system and the drain system of a thermal or nuclear power plant.
When these parts are made of carbon steel and exposed to a flow of a fluid, erosion-corrosion occurs at the surface on which the flow touches the parts. Because such power plants tend to be operated for longer periods of time, the thickness of part walls may reduce to such an extent as to cause various problems.
Therefore, the devices used in the wet steam system, the feedwater and condensate system and the drain system of a thermal or nuclear power plant have to be regularly overhauled and inspected to confirm that their wall thickness has not, due to erosion-corrosion, progressively reduced and no interference with the operation of the plant would occur.
If such development of reduced thickness appears to possibly exceed the allowable design limit, portions with developing reduced thickness are overlaid by welding, or affected parts are replaced.
Also, when the progress of thickness reduction is fast and incidental repairs by the overlaying or by the replacement of parts are frequently needed, these parts are generally produced anew using a CrMo steel or an austenitic stainless steel which has higher resistance against erosion-corrosion than carbon steel.
Here, the devices made of carbon steel constituting the wet steam system, the feedwater and condensate system and the drain system of a thermal or nuclear power plant amount to a voluminous quantity; therefore, it is very expensive to inspect each device regularly with UT (ultrasonic testing) and repair thickness-reduced portions by overlaying or replacing affected parts.
Also, if each device is to be made of austenitic stainless steel having a higher erosion-corrosion resistance than carbon steel, the cost of material would be several times as much as that of carbon steel, and therefore is uneconomical.